Here we go again!

Don’t worry Potless didn’t mean it like that It’s just that there has been a very, very long debate about where the pressure cap should go ie on the swirl tank or the header tank, hence the light-hearted response. Obviously I would not wish to express an opinion either way and I’m sure someone will be along shortly with an answer for you.

Where is Mark12345 when you need him!!!!!

This might help

Norm, I'm in Portsmouth and ducking for cover. But with nice shiney new aluminium expansion tank on mine now affixed to the side of swirl pot I have the pressure cap on the ally expansion tank and the blank on the swirl pot. Shiney photos will follow, just not yet

Mark

I am the other way around.
In 1990 when I bought a brand new s that's how my new one came from the dealer.
So I have always kept this later car the same.
Adrian

You should be able to take your old cap, either the pressure cap or the 'blank' in to any GOOD motor factor (one that has been in business a long time and has built up experience, you simply cannot expect Halfords to be able to help, our 'old' cars re not their market), and they should be able to match it from stock or within 24 hours. I would be surprised if the old cap didnt have makers marks which would help them, and the pressure cap should have the pressure marked on and be one of only a limited number of options.
Which way round to put them? From a thermodynamic theory point of view, both should work perfectly well! In one the 'bottle' works as a catch tank, in the other as an expansion vessel. I have tried both and checked the effect and both work fine on my car, PROVIDED I used them right.
Can I put a theory forward and see what the 'sage's' think? I had a much earlier TVR which did not have a 'swirlpot' and the rad had to have a header tank miles away and as high up as possible otherwise air pockets formed. Because we still have a long run of piping and a very spread out cooling system (remember the heater forms part of it and is a very long way from the radiator)we have plenty of opportunities for air locks. Furthermore we only have the original Cologne water pump to deal with a larger volume system. The swirl pot is partly there to give the system an opportunity to separate air from water, to do that it must have adequate free air space. Take the cap off the swirl pot when the engine is running but cold and watch what happens. So, fill the swirl pot to the top and it cannot work. Fill it and put the pressure cap on the swirl pot and you will have problems at first but what will happen is that as the water expands with heat it will open the cap and put water into the overflow bottle, as it cools the water will NOT be drawn back and the swirl pot has the space it needs. The system has set its own level. This system is then stable, as long as no water is lost! So if you put the pressure cap on the swirlpot, as I am pretty sure TVR intended, the swirl pot can do its job properly BUT you have no automatic compensation for any water loss. In other words, put the pressure can on the swirlpot, the system works well, but you have to check water levels regularly, as you would in any performance sports car.

If you use the bottle as an EXPANSION tank, as you would do in any modern car (which will have a simpler and smaller syatem and will NOT have a swirl pot) it should work PROVIDED you do not fill the swirl pot to the top, but you do risk the swirl pot not being able to do its job and therefore the slightly greater risk of air locks.

Come on somebody, am I on the right track or way off beam? Someone out there with racing experience or other cars that need swirl pots should be able to help here. Can we bottom this hot potato between us!?

Me? I'm going out to check the pressure cap on the swirl pot and check the water level to ensure there is some air space! I am going to add some water and see if the overflow tank has filled slightly and the swirl pot level droppe slightly after the first hot run!

I have pressure caps on the expansion tank and swirl pot. Have not lost any coolant even in the major queue ( almost an hours crawl ) in the hot summer we had a couple of weeks ago.
I do have a huge alloy radiator installed, not aware of the pressure settings of the caps though.

Blimey this takes me back. When I bought my S2 in 91 it was 6 months old. It had pressure caps on both iirc. I remember that the rubber washer was better and with a good seal indicated 'normal' temp was one notch lower on the guage. If I filled the expansion tank to the right level it would spit it out. If I didn't refill it, it wouldn't lose any more coolant.

Make of that what you will